Mechanical or chemical-mechanical planarizing processes (CMP) are used to form a substantially flat surface on microelectronic substrates such as semiconductor wafers used in the fabrication of semiconductor devices. FIG. 1 shows a planarizing apparatus 100 comprising a planarizing web medium 102 stretched over a platen 104 and a substrate holder 106 that holds the substrate 108. The planarizing medium comprises, for example, a fixed abrasive planarizing web. A fixed abrasive web comprises abrasive particles embedded within a suspension medium. In one embodiment, the planarizing apparatus has a plurality of rollers to supply, guide and collect the web-format planarizing medium. The rollers include a supply roller 110 to supply the fresh or un-used portion of the web and a take-up roller 112 to collect the worn or used portion of the web. The web is advanced across the platen such that a fresh portion of the web is introduced into the planarizing region 114 and a worn portion of the web is collected at the take-up roller 112.
During planarization, the substrate holder presses the substrate against the planarizing medium, translates and/or rotates it to planarize the substrate. It is desirable to accurately determine the endpoint of the planarization process. This is to prevent over-polish of substrates that may lead to excessive thinning, or under-polish that leaves residual material on the substrate surface, which results in defective substrates and leads to the formation of defective microelectronic components on the substrate or loss in throughput.
Conventional end-point detection (EPD) methods include optical EPD which detects the reflectivity changes of the substrate surface resulting from the removal of material from the surface of the substrate, or motor current EPD which is an indirect measurement of the frictional force changes between the substrate and the planarizing medium. Other EPD methods include thermal or acoustic EPD which also detect variations in friction during the progression of the planarization process.
However, these conventional methods do not differentiate between fresh or used portions on the planarizing web surface, which exhibit different physical properties. It is desirable to provide a more reliable method of detecting the appropriate endpoint of mechanical and/or chemical mechanical planarization processes.